| Along with society’s progress and the rapid development of science and technology,many factories and residential users home use a lot of nonlinear load,the nonlinear load in all contain the high frequency switch tube,such as,decision,GTR,IGBT,MOSFET,etc.,these will generate a lot of the opening of the switch tube on or off the harmonic current into the power grid,the grid environment.In this paper,the three-phase four-wire three-level active power filter(apf)is taken as the research object.LLCL filter circuit is widely used in active power filter because it can suppress the harmonic at switching frequency and reduce the total inductance.LLCL filter is on the basis of the original LCL filter series an inductor on the capacitor branch to form the LC high frequency attenuation branch so that it can produce series resonance at the switching frequency,so as to eliminate the high-order switching ripple and make the grid-connected current cleaner.However,LLCL filter has a resonant peak and a negative peak at the switching frequency,which will make the system unstable.Therefore,active damping and passive damping methods can be used to suppress it.Firstly,the mathematical model and working principle of the three-level active power filter(apf)with diode mid-point clamp are analyzed,several methods of extracting harmonic current are analyzed,and the theory of instantaneous reactive power and the method of extracting harmonic current based on this theory are introduced.Secondly,in order to solve the problem of the inherent imbalance of midpoint potential in three levels,the current flow direction and its influence on midpoint potential in different switching states are analyzed,and a voltage equalizing ring and a voltage stabilizing ring are designed to stabilize the midpoint potential voltage.The controller of current loop is designed.Thirdly,aiming at the resonant sharp front problem of LLCL filter,the active damping and passive damping strategies are adopted to suppress the resonant sharp front and good results are achieved.Considering the delay problem of sampling itself,the system will be unstable.Then,the measures of pre-estimation,correction and compensation are taken to make the system stable again.The capability of compensating harmonic current of different controllers was verified by MATLAB/Simulink simulation.Finally,the APF software and hardware systems are designed,and an active power filter with rated current of 100 A is built for experimental verification.And designed a touch screen and APF for communication,with touch screen to control the action of APF. |
